This invention relates to a process for producing an aluminum alloy semi-molten billet for use as a transportation unit.
Thixocasting using a semi-molten billet is an art having recently attracted considerable attention because of its advantages in which less segregation and fewer defects of casting as well as a longer service life of a mold are available when compared with conventional die-casting. There are several different processes for producing the billet. One of them is system xe2x80x9cAxe2x80x9d known as the Pechiney Alumax System in which a melt is electromagnetically and mechanically agitated at a semi-melting temperature in order to provide spheroidized primary crystals xcex1 (Al) at a billet-producing stage. Another process is system xe2x80x9cBxe2x80x9d in which a compound of Alxe2x80x94Tixe2x80x94B is added to a melt during casting in amounts greater than when usually added, and then the melt is heated up to a semi-melting temperature range, thereby yielding spheroidized primary crystals xcex1 (Al).
A further process is system xe2x80x9cCxe2x80x9d in which strain is introduced into a melt by means of extrusion/rolling, and the melt is thereafter heated up to a semi-melting temperature range so as to provide spheroidized primary crystals xcex1 (Al) as practiced in the above system xe2x80x9cB.xe2x80x9d
Such conventional processes for producing the semi-molten billet suffer from various problems. For example, system xe2x80x9cAxe2x80x9d results in a very complicated process of manufacture, and adds to manufacturing cost. System xe2x80x9cBxe2x80x9d involves addition of a large quantity of Alxe2x80x94Tixe2x80x94B, and then TiB2 settles down in a melting furnace, with consequential instability of casting quality. In system xe2x80x9cCxe2x80x9d, for strain to be introduced into the melt by means of rolling, uniform strain is difficult to provide. For strain to be introduced into the melt by means of extrusion, such extrusion usually involves a complicated manufacturing process, and further encounters a difficulty of introducing even strain into the melt. Moreover, these two methods for introducing the strain into the melt in system xe2x80x9cCxe2x80x9d require machining a worked product. This requirement hinders mass production and is a cost drawback.
In view of the above, an object of the present invention is to provide a process for producing an aluminum alloy semi-molten billet for use as a transportation unit, whereby a simpler manufacturing process and lower cost are realized, with the result that products of uniform quality are available. The above object is achievable by the foregoing process according to the present invention, the process including the steps of: producing an aluminum alloy having a composition consisting essentially of, in weight %, 0.5 or less Cu, 5.0 to 10.0 Si, 0.2 to 0.7 Mg, 0.35 or less Zn, 0.55 or less Fe, 0.5 or less Mn, 0.005 to 0.5 Ti, and the balance aluminum, introducing working strain into a melt of the aluminum alloy by means of molding flask-assisted cold forging at a strain percentage of 10 to 40%, at a working introduction velocity of 10 mm or less per second, and at a temperature of 200xc2x0 C. or lower; and, thereafter retaining such strain introduced melt at temperatures in a range of 576 to 585xc2x0 C.